Method for forming oxidation-passive layer, fluid-contacting part, and fluid feed/discharge system

ABSTRACT

A method for forming an oxidation-passive layer having high corrosion resistance to highly oxidizing materials such as ozone; a stainless steel and a titanium base alloy having corrosion resistance to an ozone containing fluid; and a fluid containing part, a process apparatus, and a fluid feed/discharge system made by using the same. The method comprises the steps of heat-treating the surface of a stainless steel or titanium-base alloy having an Al content of 0.5 percent by weight to 7 percent by weight either at 300° C. to 700° C. in a mixed gas atmosphere composed of an inert gas and 500 ppb to 1 percent H 2 O gas or 1 ppm to 500 ppm oxygen gas, or alternatively at 20° C. to 300° C. in a mixed gas atmosphere composed of an oxygen gas and at least 100 ppm ozone gas to form an oxidation-passive layer containing an aluminum oxide or a titanium oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming an oxidationpassive layer, to a fluid-contacting part, and to a fluid feed system.In greater detail, the present invention relates to a method for formingan oxidation passive layer having a layer chiefly comprising aluminumoxides on the surface of stainless steel, a method for forming anoxidation passive layer chiefly comprising titanium oxides on a titaniumbase alloy surface, stainless steel or titanium base alloy having suchpassive layers formed thereon, and a fluid-contacting part and fluidfeed system having parts in contact with a fluid (gas or liquid)employing these stainless steel and titanium materials.

2. Description of the Related Art

Chromium oxide passive layers are highly resistant to corrosion byvarious semiconductor manufacturing process gases. Moreover, theoutgassing properties thereof are extremely superior, allowing suchlayers to be employed in vacuum devices, reduced-pressure devices, andgas supply pipes which require a high degree of cleanliness. Thesechromium oxide passive layers may also be used in supply pipes forultrapure water and the like.

Recently, the high oxidizing power of ozone has been used in varioustechnologies, such as cleaning of silicon substrate, ashing, andlow-temperature CVD oxidation to develop highly efficient and integrateddevice.

However, in ozone supply piping materials, fluorine resins such as PVDFor the like, which are commonly employed in wet systems, and SUS316 andthe like, which is commonly employed in gas systems, are markedlycorroded by ozone. This represents a source of contamination, so that itis impossible to use such materials. Furthermore, as the ozoneconcentration increases, even the oxidation of the chromium oxidepassive layers described above from Cr₂O₃ to CrO₃ occurs as a result ofthe oxidizing power of the ozone. Therefore, it becomes impossible tomaintain a high state of cleanliness in the piping and the atmosphereand the like.

In light of the above circumstances, the present invention has as anobject thereof to provide a method for forming an oxidation passivelayer which is highly resistant to corrosion by strongly oxidizingsubstances such as ozone.

Furthermore, it is an object of the present invention to providestainless steel and titanium base alloys which are strongly resistant tocorrosion by fluids containing ozone, as well as to providefluid-contacting parts, process apparatuses, and fluid feed systems anddischarge systems employing these corrosion resistent materials.

SUMMARY OF THE INVENTION

In a method for forming oxidation passive layers of the presentinvention, a stainless steel surface containing Al in an amount within arange of 0.5 percent by weight to 7 percent by weight is heat treated ata temperature within a range of 300° C. to 700° C. in a mixed gasatmosphere of an inert gas and 500 ppb to 1 percent H₂O gas, andthereby, an oxidation passive layer containing aluminum oxides isformed.

Furthermore, in another method for forming oxidation passive layers inaccordance with the present invention, a stainless steel surfacecontaining Al in an amount within a range of 0.5 percent by weight to 7percent by weight is polished to a Rmax of 0.7 micrometers or less, andbaked in an inert gas atmosphere, whereby moisture is removed from thesurface of the stainless steel, and subjected to heat treatment at atemperature within a range of 300° C. to 700° C. in a mixed gasatmosphere of an inert gas and 500 ppb to 1 percent H₂O gas, andthereby, an oxidation passive layer containing aluminum oxides isformed.

In another method for forming oxidation passive layers in accordancewith the present invention, a stainless steel surface containing Al inan amount within a range of 0.5 percent by weight to 7 percent by weightis subjected to heat treatment at a temperature within a range of 300°C. to 700° C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500ppm of oxygen gas, and thereby, an oxidation passive layer containingaluminum oxides is formed.

In another method for forming oxidation passive layers in accordancewith the present invention, a stainless steel surface containing Al inan amount within a range of 0.5 percent by weight to 7 percent by weightis polished to a Rmax of 0.7 micrometers or less, and then, baked in aninert gas atmosphere, whereby moisture is removed from the stainlesssteel surface, and then heat treatment is conducted at a temperaturewithin a range of 300° C. to 700° C. in a mixed gas atmosphere of aninert gas and 1 ppm to 500 ppm of oxygen gas, and thereby, an oxidationpassive layer containing aluminum oxides if formed.

In the present invention, it is preferable that hydrogen gas be added tothe mixed gas in an amount of 10 percent or less.

In another method for forming oxidation passive layers in accordancewith the present invention, a stainless steel surface containing Al inan amount within a range of 0.5 percent by weight to 7 percent by weightis heat treated at a temperature within a range of 20° C. to 300° C. ina mixed gas atmosphere containing oxygen gas and at least 100 ppm ofozone gas, and thereby, an oxidation passive layer containing aluminumoxides is formed.

In a further method for forming oxidation passive layers in accordancewith the present invention, a stainless steel surface containing Al inan amount within a range of 0.5 percent by weight to 7 percent by weightis polished to a Rmax of 0.7 micrometers or less, and baked in an inertgas atmosphere, whereby moisture is removed from the stainless steelsurface, and then this is subjected to heat treatment at a temperaturewithin a range of 20° C. to 300° C. in a mixed gas atmosphere containingoxygen gas and at least 100 ppm of ozone gas, and thereby, an oxidationpassive layer containing aluminum oxides is formed.

In a further embodiment, it is characteristic that nitrogen gas is addedin an amount of 10 percent or less to the mixed gas containing ozone gasas described above.

In the methods for forming oxidation passive layers in accordance withthe present invention, it is preferable that the amount of Al containedin the stainless steel be within a range of 3 percent by weight to 6percent by weight.

Additionally, in yet another embodiment, it is characteristic that theoxidation passive layer chiefly comprises a mixed oxide layer ofaluminum oxides and chromium oxides.

In another method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is heattreated at a temperature within a range of 300° C. to 700° C. in a mixedgas atmosphere of an inert gas and 500 ppb to 1 percent H₂O gas, andthereby, an oxidation passive layer comprising titanium oxides isformed.

In a further method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is polished toa Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere,whereby moisture is removed from the titanium base alloy surface, andthen heat treatment is conducted at a temperature within a range of 300°C. to 700° C. in a mixed gas atmosphere of an inert gas and 500 ppb to 1percent H₂O, and thereby, an oxidation passive layer comprising titaniumoxides is formed.

In another method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is heattreated at temperature within a range of 300° C. to 700° C. in a mixedgas atmosphere of an inert gas and 1 ppm to 500 ppm of oxygen gas, andthereby, an oxidation passive layer comprising titanium oxides isformed.

In a further method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is polished toa Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere,whereby moisture is removed from the surface of the stainless steel, andsubsequently, by heat treatment at a temperature within a range of 300°C. to 700° C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500ppm of oxygen gas, thereby an oxidation passive layer comprisingtitanium oxides is formed. In the above heat treatment, it is preferablethat hydrogen gas be in an amount of 10 percent or less.

In another method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is subjectedto heat treatment at temperature within a range of 20° C. to 300° C. ina mixed gas atmosphere of oxygen gas and 100 ppm or more of ozone gas,and thereby, an oxidation passive layer comprising titanium oxides isformed.

In another method for forming oxidation passive layers in accordancewith the present invention, a titanium base alloy surface is polished toa Rmax of 0.7 micrometers or less and baked in an inert gas atmosphere,whereby moisture is removed from the titanium base alloy surface, andheat treatment is conducted at a temperature within a range of 20° C. to300° C. in a mixed gas atmosphere of oxygen gas and 100 ppm or moreozone gas, thereby an oxidation passive layer comprising titanium oxidesis formed. In a further embodiment, 10 percent or less of nitrogen gasis added to the mixed gas.

In the present invention, the titanium base alloy contains 99 percent byweight or more of Ti, or alternatively, contains 99 percent by weight ormore of Ti, 0.05 percent by weight or less of Fe, 0.03 percent by weightor less of C, 0.03 percent by weight or less of Ni, 0.03 percent byweight or less of Cr, 0.005 percent by weight or less of H, 0.05 percentby weight or less of O, and 0.03 percent by weight or less of N.

The stainless steel of the present invention has an oxidation passivelayer having a thickness of 3 nm or more and chiefly containing aluminumoxides at the outermost surface thereof. Alternatively, an oxidationpassive layer having a thickness of 3 nm or more and chiefly comprisingaluminum oxides at the outermost surface is formed on a surface polishedto a Rmax of 0.7 micrometers or less.

The amount of Al contained in the stainless steel is preferably within arange of 0.5 percent by weight to 7 percent by weight, and morepreferably within a range of 3 percent by weight to 6 percent by weight.

The passive layer comprises mixed oxides of, chiefly, aluminum oxidesand chromium oxides.

The titanium base alloy of the present invention has an oxidationpassive layer having a thickness of 3 nm or more and comprising titaniumoxides at the outermost surface formed thereon. Alternatively, anoxidation passive layer having a thickness of 3 nm or more andcomprising titanium oxides at the outermost surface thereof is formed ona surface polished to a Rmax of 0.7 micrometers or less.

The titanium base alloy contains 99 percent or more of Ti, oralternatively, contains 99 percent or more of Ti, 0.05 percent by weightor less of Fe, 0.03 percent by weight or less of C, 0.03 percent byweight or less of Ni, 0.03 percent by weight or less of Cr, 0.005percent by weight or less of H, 0.05 percent by weight or less of O, and0.03 percent by weight or less of N.

The fluid-contacting part of the present invention has parts in contactwith fluid which comprises stainless steel or titanium base alloy inaccordance with the present invention.

The process apparatus of the present invention has parts in contact withfluid which comprise stainless steel or titanium base alloy inaccordance with the present invention.

The fluid feed system of the present invention has parts in contact withfluid which comprise stainless steel or titanium base alloy inaccordance with the present invention.

The fluid feed system of the present invention has parts in contact withfluid which comprise stainless steel or titanium base alloy inaccordance with the present invention.

The discharge system of the present invention has parts in contact withfluid which comprise stainless steel or titanium base alloy inaccordance with the present invention.

A method for forming oxidation passive layers on stainless steel will beexplained as an example of a method for forming oxidation passive layersin accordance with the present invention.

The stainless steel employed contains 0.5 percent by weight to 7 percentby weight of Al. At amounts of less than 0.5 percent, a passive layerhaving high corrosion resistance cannot be formed, and if the amount isin excess of 7 percent, intermetallic oxides are formed and it isimpossible to obtain a stable passive layer. It is particularlypreferable that the amount of Al contained be within a range of 3percent by weight to 6 percent by weight, and in this range, thealuminum oxide component ratio is further increased, and it is possibleto form an oxidation passive layer having superior corrosion resistancewith respect to ozone.

It is preferable that the surface of the stainless steel be polished soas to achieve a surface roughness Rmax of 0.7 micrometers or less, usingelectropolishing, composite electropolishing, polishing with polishinggranules, buff polishing, or the like. By making the surface smooth, itis possible to reduce the amount of emitted gas, to increase theadhesion, and to suppress the generation of particulate matter so as toform a minute oxide layer. When the surface roughness is reduced, itbecomes more difficult to form an oxidation passive layer, so that thesurface roughness may be determined along with the formationtemperature, the atmospheric concentration, the time, and the like, inaccordance with the desired layer thickness and layer characteristics.

The following first through third oxidation methods are encompassed inthe oxidation method of the present invention.

First, the first method is one in which heat treatment (300° to 700° C.)is conducted in an inert gas atmosphere containing a trace amount ofmoisture (500 ppb to 1 percent).

In the present method, as the moisture concentration increases, there isa tendency for the passive layer formation speed to increase. If theamount of moisture is less than 500 ppb, it is difficult to form apassive layer having aluminum oxide as a chief component thereof.Furthermore, the layer formation rate is extremely slow, so that this isnot suitable for practical application. On the other hand, if the amountof moisture is in excess of 1 percent, although this is also related tothe generation temperature, it becomes difficult to form a minutepassive layer having a high degree of resistance to ozone.

As the heat treatment temperature increases, the rate of layer formationincreases. At temperatures lower than 300° C., there is almost noformation of the passive layer and practical application is impossible.At temperatures in excess of 700° C., surface irregularities areproduced, and the resistance to ozone also decreases. Therefore, theheat treatment temperature is set within a range of 300° C. to 700° C.

It is preferable that hydrogen gas be added to the inert gas, asdescribed above, in an amount of 10 percent or less, and particularlypreferably in an amount of 3 percent to 10 percent. By adding thehydrogen gas, it is possible to reduce the proportion of iron oxide inthe oxidation passivation layer, and to form a passive layer havinggreater resistance to ozone.

The second oxidation method is one in which heat treatment (300° C. to700° C.) is conducted in an inert gas atmosphere containing a traceamount (1 ppm to 500 ppm) of oxygen.

As the oxygen concentration increases, there is a tendency for the rateof generation of the passive layer to increase, and in the same manneras in the first method described above, in order to efficiently obtain apassive layer having a high degree of resistance to ozone, it isnecessary that the oxygen concentration be within a range of 1 ppm to500 ppm. Furthermore, for the same reason as in the first method, it ispreferable that hydrogen gas be added to the inert gas in an amount of10 percent or less.

The third method is one in which treatment (20° C. to 300° C.) isconducted in the presence of oxygen containing at least 100 ppm ofozone.

In this method, it is possible to produce an oxidation passive layer atlow temperatures, and moreover, it is possible to form an oxidationpassive layer having high resistance to ozone. Oxygen gas containing 100ppm or more of ozone may be obtained by subjecting pure oxygen gas or agas containing oxygen gas to an electric discharge by means of silentdischarge. In such a case, it is preferable that 10 percent or less ofnitrogen gas (preferably within a range of 4 percent to 6 percent) beadmixed.

If the treatment temperature is in excess of 300° C., the ozone willbreak down and the iron oxide component will increase, and theresistance to ozone will decrease. Therefore, this temperature is set at300° C. or below. Furthermore, if the treatment temperature is reducedto approximately room temperature, the formation of the layer isdramatically slowed, so that it is preferable to set the ozoneconcentration at, for example, 7 percent.

In the first through third methods described above, prior to conductingoxidation treatment, it is preferable that the surface to be subjectedto oxidation treatment be polished to a Rmax of 0.7 micrometers or lessin advance, and that this then be subjected to baking in an inert gasatmosphere (preferably within a range of 200° C. to 600° C.). By meansof this preprocessing, the cleanliness of the layer is increased, andthe resistance to ozone is further increased.

Next, a method for forming oxidation passive layers on a titanium basealloy will be explained.

Fundamentally, this is identical to the case in which stainless steel isemployed. In other words, heat treatment is conducted in an inert gasatmosphere containing a trace amount (500 ppb to 1 percent) of moistureor a trace amount (1 ppm to 500 ppm) of oxygen, and thereby, it ispossible to form an oxidation passive layer having high resistance toozone which has titanium oxides as the chief component thereof.

Ti occludes hydrogen gas and becomes brittle, so that normally Ti is notbrought into contact with hydrogen. However, in the present invention,event if 10 percent or less of hydrogen is added when the Ti isoxidized, the titanium does not become brittle as a result of thehydrogen. On the contrary, a minute and strong passive layer is formed.

Furthermore, if treatment (at 20° C. to 300° C.) employing oxygen gascontaining at least 100 ppm of ozone is carried out as well, it ispossible to form an oxidation passive layer having high resistance toozone which has titanium oxides as the chief component thereof.

Examples of inert gases which were preferably employed in the presentinvention include, for example, N₂ gas, Ar gas, and the like.

The stainless steel of the present invention has a oxidation passivelayer having a thickness of 3 nm or more chiefly comprising aluminumoxides at the outermost surface thereof. Stainless steel having anoxidation passive layer containing chiefly aluminum oxides of athickness of 3 nm exhibits extremely high resistance to ozone. It ispreferable that the oxidation passive layer containing chiefly aluminumoxides and having a thickness of 3 nm be formed on a stainless steelsurface having a Rmax of 0.7 micrometers or less; the resistance toozone corrosion of such stainless steel is further increased.

The stainless steel base material of the present invention contains Alin an amount within a range of 0.5 percent by weight to 7 percent byweight, and more preferably within a range of 3 percent by weight to 6percent by weight. By employing such stainless steel, it is easilypossible to form an oxidation passive layer having aluminum oxides as achief component thereof and having a thickness of 3 nm or more.

The titanium base alloy of the present invention has an oxidationpassive layer having a thickness of 3 nm or more and chiefly comprisingtitanium oxides at the outermost surface thereof. The titanium basealloy having a 3 nm oxidation passive layer containing chiefly titaniumoxides exhibits extremely high resistance to corrosion with respect toozone. It is preferable that the oxidation passive layer containingchiefly titanium oxides and having a thickness of 3 nm be formed on thesurface of stainless steel having a Rmax of 0.7 micrometers or less,whereby, the resistance to corrosion by ozone of such stainless steel isfurther increased.

The titanium base alloy of the present invention preferably contains Tiin an amount of 99 percent by weight or greater. More preferably, thistitanium base alloy contains the further impurities of Fe in an amountof 0.05 percent by weight or less, C in an amount of 0.03 percent byweight or less, Ni in an amount of 0.03 percent by weight or less, Cr inan amount of 0.03 percent by weight or less, H in an amount of 0.005percent by weight or less, O in an amount of 0.05 percent by weight orless, and N in an among of 0.03 percent by weight or less. By employingsuch a titanium base alloy, it is easily possible to form an oxidationpassive layer having titanium oxides as a chief component thereof andhaving a thickness of 3 nm or more.

The oxidation passive layer formed in accordance with the presentembodiment described above has anti-corrosive properties with respect tocorrosive gases such as hydrogen chloride gas or the like, and hasoutgassing characteristics, which are superior, similarly to those ofchromium oxide passive layers. In addition, such layers are extremelystable even with respect to fluids containing strongly oxidizingsubstances such as ozone. Accordingly, the stainless steel or titaniumbase alloy of the present invention may be used in process apparatusessuch as vacuum or depressurized apparatuses, which require a highlyclean atmosphere; as fluid-contacting parts of various types of gas orultrapure water supply pipe systems, such as valves, filters, junctions,and the like; and in discharge systems, such as fluid feed systems,pumps, and the like. In addition, the stainless steel or titanium basealloy may be appropriately employed even with respect to cases in whichfluids are employed which contain ozone and the like. Furthermore, it isa simple matter to employ the stainless steel of the present inventionas material for lines having a diameter of few micrometers, andfurthermore, an oxidation passive layer may be formed on the surfacethereof, so that use in gas filters and the like is particularlyappropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph showing the profile in the direction of depth of theconstituent atoms of an oxidation passive layer on stainless steel;

FIG. 2 is a graph showing the relationship between the profile in thedirection of depth of the constituent atoms of an oxidation passivelayer on stainless steel and the oxidation temperature;

FIG. 3 is a graph showing the relationship between the profile in thedirection of depth of the constituent atoms of an oxidation passivelayer on stainless steel and the moisture concentration used foroxidation;

FIG. 4 is a graph showing the relationship between the profile in thedirection of depth of the constituent atoms of a passive layer beforeand after impregnation in ozonated water;

FIG. 5 is a graph showing the changes in the depth profile of theconstituent atoms of a passive layer before and after exposure to ozonegas;

FIG. 6 is a graph showing the changes in the depth profile of theconstituent atoms before and after oxidation passive layer treatment ofa titanium base alloy;

FIG. 7 shows the ESCA spectra of a titanium oxide sintered body and anoxidation passive layer; and

FIG. 8 is a graph showing the changes in the depth profile of theconstituent atoms before and after impregnation in ozonated water.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

In embodiment one of the present invention, the austenitic stainlesssteels (SA7 through SA9) having an Al content of approximately 5 percentby weight shown in Table 1 were electropolished, and the surfaceroughness Rmax thereof was set to 0.3 micrometers.

TABLE 1 Ni Cr Al C Si Mn P S Mo Cu Nb N SA7 26.02 18.70 5.29 0.018 0.120.16 0.018 <0.001 0.01 0.01 0.20 0.026 SA8 31.07 18.60 5.15 0.018 0.120.16 0.017 <0.001 <0.01 0.01 0.20 0.022 SA9 35.87 18.33 5.11 0.015 0.120.15 0.015 <0.001 0.01 0.01 0.18 0.019 SUS316L 15.10 17.15 — 0.001 0.010.01 0.004 0.001 2.76 — — 0.0073

The samples described above (SA7, 8, 9) were inserted into an oxidizingtreatment furnace, and the temperature within was raised from roomtemperature to 600° C. in a period of 30 minutes while introducing Argas having an impurity concentration of 1 ppb into the furnace, andbaking was conducted for a period of one hour at this temperature, andthe moisture adhering to the surface of the sample was removed.

After the completion of baking, the temperature was maintained and thegas was switched to a treatment gas in which 10 percent hydrogen gas and100 ppm of moisture were present in an Ar gas atmosphere, and heattreatment was conducted for a period of 6 hours.

FIGS. 1(a) and (b) show ESCA analyses before and after oxidation passivelayer formation treatment using SA8 as an example. In the figures, thevertical axis indicates the composition of each constituent atom, whilethe horizontal axis indicates the etching time by means of ions, whichcorresponds to the depth of the surface. Here, the etching rate was 7.0nm/min in silicon conversion.

Although not shown in the figures, essentially identical results wereobtained for SA7 and SA9.

As is clear from FIGS. 1(a) and (b), the surface of the stainless steeltreated in accordance with the conditions described above has a passivelayer comprising chiefly aluminum oxides formed thereon to a depth ofapproximately 60 nm. The thickness of the passive layer is, in FIGS.1(a) and (b), the intersection point between Al and Fe.

In embodiment two of the present invention, an SA8 sample was treated inthe manner of embodiment one, with the exception that the moistureconcentration was set to 1 ppm and the temperature of the oxidationtreatment was varied in a number of ways, and oxidation passive layerswere formed. An example of the ESCA analyses measured with respect to asample of the oxidation passive layers formed is shown in FIG. 2. InFIG. 2, (a) indicates the state before treatment, (b) indicatestreatment at 500° C., (c) indicates treatment at 550° C., and (d)indicates treatment at 600° C.

As is clear from FIG. 2, as the treatment temperature increases, thethickness of the layer in which Al oxides predominate increases.Although not shown in the figures, irregularities occurred to thesurface of the passive layer when the temperature exceeded 600° C., andwhen the temperature was in excess of 700° C., these irregularities werepronounced. On the other hand, at 300° C., although the qualities of thelayer remain essentially unchanged, the rate of growth of the passivelayer was slowed, and was one-tenth that of the case in which atemperature of 500° C. was employed.

In embodiment three of the present invention, a SA7 sample was subjectedto oxidation passive layer formation under oxidation conditionsidentical to those of embodiment one, with the exception that themoisture concentration was varied. The ESCA analyses are shown in FIG. 3with respect to a portion of the samples of the passive layers formed.In FIG. 3, (a) indicates the state prior to treatment, (b) indicatestreatment at 0.5 ppm, (c) indicates treatment at 1 ppm, and (d)indicates treatment at 10 ppm.

As can be seen from FIGS. 3(a) through (d), as the moistureconcentration increased, the thickness of the passive layer alsoincreased.

In embodiment four of the present invention, the oxidation passive layerof the embodiment 1 (SA7) and a chromium oxide oxidation passive layerwere evaluated for resistance to ozonated ultrapure water.

The chromium oxide passive layer was formed by oxidation using a methodabsolutely identical to that of embodiment one using SUS316L having thecomposition shown in Table 1, and when the profile in the depthdirection of the chromium oxide passive layer was measured By ESCA, itwas determined that a passive layer comprising chromium oxide was formedwith a depth of 20 nm.

In the evaluation method, a sample was immersed in ultrapure watercontaining an ozone concentration of 2 ppm. After immersion, the samplewas removed, and an observation of the surface was conducted, whereuponit was determined that the chromium oxide passive layer disappearedafter three days, while the aluminum oxide passive layer of embodimentone remained unchanged after 10 days. An observation using scanningelectron microscopy confirmed that there was absolutely no change in thesurface.

In embodiment five of the present invention, the stainless steelindicated by SA7 of Table 1 was inserted into an oxidation treatmentfurnace, and the temperature therewithin was raised from roomtemperature to 600° C. over a period of 30 minutes while introducing Argas having an impurity concentration of 1 ppb into the furnace, bakingwas conducted for a period of one hour at this temperature, and theadsorbed moisture was removed from the surface of the sample.

After the completion of baking, the same temperature was maintained andthe gas was exchanged from a treatment gas comprising 10 percenthydrogen gas and 1000 ppm of moisture in an Ar gas atmosphere, and heattreatment was conducted for a period of 6 hours.

The SA7 having a passive layer formed thereon was immersed for a periodof 10 days in water containing 3 ppm of ozone, and the surface wasobserved using scanning electron microscopy and ESCA before and afterimmersion. In the electron microscopy, no change was observed in thesurface; however, it can be seen from the ESCA analyses before and afterimmersion which were shown in FIGS. 4(a) and (b) that there was someslight erosion of the passive layer.

In embodiment six of the present invention, stainless steel having thecomposition indicated by SA8 of Table 1 was introduced into an oxidationtreatment furnace, the temperature therewithin was raised from roomtemperature to 550° C. over a period of 30 minutes while introducing Argas having an impurity concentration of 1 ppm into the furnace. Thistemperature was maintained while exchanging the gas for a treatment gasin which hydrogen gas was contained in an amount of 10 percent andmoisture was contained in an amount of 10 ppm in an Ar gas atmosphere.Heat treatment was conducted for a period of 6 hours.

The SA7 having a passive layer formed thereon was subjected to a flow ofoxygen containing 7 percent ozone gas at a rate of 1 L/min and at roomtemperature and the effect of the ozone gas was determined by means ofESCA. The results thereof are shown in FIG. 5. In FIG. 5, (a) indicatesthe state prior to ozone gas exposure, while (b) indicates the stateafter exposure.

As can be seen from FIGS. 5(a) and (b), the oxidation passive layer ofthe present embodiment was completely stable even with respect to a highconcentration of ozone gas.

In embodiment seven of the present invention, stainless steel having thecomposition of SA8 was prepared, with the exception that the amount ofAl contained was varied, and oxidation passive layers were formed in themanner of embodiment one, and the resistance to ozone and surfaceroughness thereof were evaluated. The results thereof are shown in Table2.

TABLE 2 Al (%) 0.1 0.5 3 6 7 8 Ozone X ≧ ∘ ∘ ∘ ∘ Resistance Surface ≧ ∘∘ ∘ ≧ X Roughness

From Table 2, it can be seen that when the amount of Al contained waswithin a range of 3 percent by weight to 6 percent by weight, and boththe resistance to ozone and the surface roughness were superior.

In embodiment eight of the present invention, SA8 was inserted into anoxidation treatment furnace, the temperature therewithin was raised fromroom temperature to 600° C. over a period of 30 minutes whileintroducing Ar gas having an impurity concentration of 1 ppb into thefurnace, and baking was conducted at this temperature for a period ofone hour to remove adsorbed moisture from the sample surface.

After the conclusion of baking, this temperature was maintained, andoxygen gas was introduced at a concentration of 1 ppm, 10 ppm, and 500ppm, and 10 percent hydrogen gas was introduced into the Ar atmosphere,and heat treatment was conducted for a period of 6 hours.

When the oxidation passive layer was observed by means of ESCA, it wasconfirmed that oxidation passive layers having aluminum oxides as thechief component thereof were formed at thicknesses of 7 nm, 10 nm, and20 nm, respectively.

In embodiment 9 of the present invention, SA8 was introduced into anoxidation treatment furnace, the temperature therewithin was raised fromroom temperature to 100° C. over a period of 10 minutes whileintroducing Ar gas having an impurity concentration of 5 ppb into thefurnace, and oxygen gas (containing 4 percent nitrogen gas) containingozone in an amount of 100 ppm was introduced from an ozone generatingapparatus (the SG-01AH produced by Sumitomo Seimitsu Kogyo K. K.), andthis was subjected to oxidation treatment for a period of 6 hours.

When the oxidation passive layer was observed by means of ESCA, it wasdetermined that an oxidation passive layer having aluminum oxides as achief component thereof was formed to a thickness of 10 nm.

In embodiment ten of the present invention, the Ti material contained 99percent by weight of Ti, and as impurities, contained 0.05 percent byweight of Fe, 0.03 percent by weight of C, 0.03 percent by weight of Ni,0.03 percent by weigh of Cr, 0.005 percent by weight of H, 0.05 percentby weight of O, and 0.03 percent by weight of N, and this was polishedusing polishing granules and the surface roughness Rmax thereof was setto 0.7 micrometers.

The sample described above was inserted into an oxidation treatmentfurnace, and the temperature therewithin was raised from roomtemperature to 500° C. over a period of 30 minutes while introducing Argas having an impurity concentration of 1 ppb into the furnace, bakingwas conducted at this temperature for a period of one hour, and theadsorbed moisture was removed from the sample surface. After thecompletion of baking, the temperature was maintained and the gas wasexchanged for a treatment gas in which hydrogen gas was contained in anamount of 10 percent and moisture was contained in an amount of 100 ppmin a Ar gas atmosphere, and heat treatment was conducted for a period ofone hour.

FIGS. 6(a) and (b) show ESCA analyses before and after treatment. Asshown in FIG. 6, the surface of the titanium material treated under theconditions described above had a passive layer comprising titaniumoxides formed thereon, and the thickness thereon was found to be 50 nm.The etching rate was 7 nm/second in silicon conversion.

Furthermore, in FIG. 7, the ESCA spectrum (b) of the oxidation passivelayer and the spectrum (a) of the titanium oxide sintered body arecompared. As is clear from FIGS. 7(a) and (b), the titanium oxide of theoxidation passive layer formed in the present embodiment is essentiallythe same as that of the titanium oxide sintered body.

Next, the oxidation passive layer formed in the present embodiment wasimmersed for a period of one month in water containing 12 ppm of ozonetogether with untreated titanium material. ESCA analyses before andafter immersion are shown in FIG. 8.

It can be seen that the titanium material itself was deeply oxidized incomparison with the state prior to immersion, and furthermore, in thecase of the oxidation passive layer, the same profile was observed to anetching time of 3.5 minutes, and it can thus be seen that the surfacedid not change as a result of immersion.

In embodiment eleven of the present invention, the Ti material employedin embodiment ten was inserted into an oxidation treatment furnace, thetemperature therewithin was raised from room temperature to 500° C.while introducing Ar gas having an impurity concentration of 5 ppb intothe furnace, baking was conducted at this temperature for a period ofone hour, and the adsorbed moisture was removed from the surface of thesample.

After the conclusion of baking, this temperature was maintained, andoxygen gas was introduced into the Ar atmosphere at concentrations of 1ppm, 10 ppm, and 500 ppm, and 10 percent hydrogen gas was introduced,and heat treatment was conducted for a period of one hour.

When the oxidation passive layers were observed by means of ESCA, it wasdetermined that oxidation passive layers comprising titanium oxides wereformed having depths of 10 nm, 20 nm, and 70 nm, respectively.

In embodiment twelve of the present invention, the Ti material employedin embodiment ten was introduced into an oxidation treatment furnace,the temperature therewithin was raised from room temperature to 100° C.over a period of 10 minutes while introducing Ar gas having an impurityconcentration of 5 ppm into the furnace, and oxygen gas (containing 5percent nitrogen gas) containing 100 ppm of ozone was introduced from anozone generating apparatus (the SG-01AH produced by Sumitomo SeimitsuKogyo K. K.), and heat treatment was conducted for a period of 6 hours.

When the oxidation passive layer was observed by means of ESCA, it wasdetermined that an oxidation passive layer comprising titanium oxidehaving a thickness of 40 nm was formed.

By means of the forming method for oxidation passive layers inaccordance with the present invention, an oxidation passive layer havingaluminum oxides as a chief component thereof can be easily and stablyformed on stainless steel or alternatively, and oxidation passive layercomprising titanium oxides can be easily and stably formed on a titaniumbase alloy.

The oxidation passive layer formed in accordance with the presentinvention is stable with respect to strongly oxidizing substances suchas ozone and the like.

Accordingly, it is possible to provide the stainless steel and titaniumbase alloy of the present invention as stable and highly clean materialsfor feed systems and treatment apparatuses for washing and ozone gastreatment and the like which employ ozone. The stainless steel andtitanium alloy of the present invention have applications in themanufacturing processes of highly functional and highly integrateddevices.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A method for forming oxidation passive layers,comprising; subjecting stainless steel containing Al in an amount withina range of 0.5 percent by weight to 7 percent by weight to heattreatment within a range of 300° C. to 700° C. in a mixed gas atmosphereof an inert gas and oxygen gas in an amount within a range of 1 ppb to500 ppm, thereby forming an oxidation passive layer containing aluminumoxides on a surface of the stainless steel.
 2. The method for formingoxidation passive layers according to claim 1, wherein the mixed gasfurther comprises hydrogen gas in an amount of 10 percent or less. 3.The method for forming oxidation passive layers according to claim 2,wherein the amount of Al contained in the stainless steel is within arange of 3 percent by weight to 6 percent by weight.
 4. The method forforming oxidation passive layers according to claim 1, wherein theamount of Al contained in the surface of stainless steel is within arange of 3 percent by weight to 6 percent by weight. 5.The method forforming oxidation passive layers according to claim 1, wherein theoxidation passive layer comprises a mixed oxide layer of aluminum oxidesand chromium oxides.
 6. A method for forming oxidation passive layers,comprising: polishing a surface of stainless steel containing Al in anamount within a range of 0.5 percent by weight to 7 percent by weight toachieve a Rmax of 0.7 micrometers or less; baking the stainless steel inan inert gas atmosphere, thereby removing moisture from the surface ofthe stainless steel; and subjecting the stainless steel to heattreatment at a temperature within a range of 300° C. to 700° C. in amixed gas atmosphere of an inert gas and oxygen in an amount within arange of 1 ppb to 500 ppm, thereby forming an oxidation passive layercontaining aluminum oxides on the surface of the stainless steel.
 7. Amethod for forming oxidation passive layers, comprising: subjectingstainless steel containing Al in an amount within a range of 0.5 percentby weight to 7 percent by weight to heat treatment within a range of 20°C. to 300° C. in a mixed gas atmosphere of oxygen gas and 100 ppm ofozone gas, thereby forming an oxidation passive layer containingaluminum oxides on a surface of said stainless steel.
 8. The method forforming oxidation passive layers according to claim 7, wherein the mixedgas further comprises nitrogen gas in an amount of 10 percent or less.9. The method for forming oxidation passive layers according to claim 7,wherein the amount of Al contained in the stainless steel is within arange of 3 percent by weight to 6 percent by weight.
 10. The method forforming oxidation passive layers according to claim 7, wherein theoxidation passive layer comprises a mixed oxide layer of aluminum oxidesand chromium oxides.
 11. A method for forming oxidation passive layers,comprising: polishing a surface of stainless steel containing Al in anamount within a range of 0.5 percent by weight to 7 percent by weight toachieve an Rmax of 0.7 micrometers or less; baking the stainless steelin an inert gas atmosphere, thereby removing moisture from the surfaceof the stainless steel; and subjecting the stainless steel to heattreatment at a temperature within a range of 20° C. to 300° C. in amixed gas atmosphere of oxygen gas and 100 ppm of ozone gas, therebyforming an oxidation passive layer containing aluminum oxides on thesurface of the stainless steel.
 12. A method for forming oxidationpassive layers, comprising: polishing a surface of a titanium base toachieve a Rmax of 0.7 μm or less; baking the titanium base alloy in aninert gas atmosphere, thereby removing moisture from the surface of thetitanium base alloy; and subjecting the titanium base alloy to heattreatment at a temperature within a range of 300° C. to 700° C. in amixed gas atmosphere of an inert gas and H₂O in an amount within a rangeof 500 ppb to 1 percent, thereby forming an oxidation passive layercomprising titanium oxides.
 13. A method for forming oxidation passivelayers, comprising: a subjecting a surface of a titanium base alloy toheat treatment within a range of 300° C. to 700° C. in a mixed gasatmosphere of an inert gas and oxygen gas in an amount within a range of1 ppb to 500 ppm, thereby forming an oxidation passive layer comprisingtitanium oxides.
 14. The method for forming oxidation passive layersaccording to claim 13, wherein the titanium base alloy contains Ti in anamount of 99 percent by weight or more, Fe in an amount of 0.05 percentby weight or less, C in an amount of 0.03 percent by weight or less, Niin an amount of 0.03 percent by weight or less, Cr in an amount of 0.03percent by weight or less, H in an amount of 0.005 percent by weight orless, O in an amount of 0.05 percent by weight or less, and N in anamount of 0.03 percent by weight or less.
 15. A method for formingoxidation passive layers, comprising: polishing a surface of titaniumbase alloy to achieve a Rmax of 0.7 micrometers or less; baking thetitanium base alloy in an inert gas atmosphere, thereby removingmoisture from the surface of the titanium base alloy; and subjecting thetitanium base alloy surface to heat treatment at a temperature within arange of 300° C. to 700° C. in a mixed gas atmosphere of an inert gasand oxygen in an amount within a range of 1 ppb to 500 ppm, therebyforming an oxidation passive layer comprising titanium oxides.
 16. Amethod for forming oxidation passive layers, comprising: subjecting asurface of titanium base alloy to heat treatment within a range of 20°to 300° C. in a mixed gas atmosphere of oxygen gas and 100 ppm of ozonegas, thereby forming an oxidation passive layer comprising titaniumoxides.
 17. The method for forming oxidation passive layers according toclaim 16, wherein the mixed gas further comprises hydrogen gas in anamount of 10 percent or less.
 18. The method for forming oxidationpassive layers according to claim 16, wherein the titanium base alloycontains Ti in an amount of 99 percent by weight or more.
 19. The methodfor forming oxidation passive layers according to claim 16, wherein thetitanium base alloy contains Ti in an amount of 99 percent by weight ormore, Fe in an amount of 0.05 percent by weight or less, C in an amountof 0.03 percent by weight or less, Ni in an amount of 0.03 percent byweight or less, Cr in an amount of 0.03 percent by weight or less, H inan amount of 0.005 percent by weight or less, O in an amount of 0.05percent by weight or less, and N in an amount of 0.03 percent by weightor less.
 20. A method for forming oxidation passive layers, comprising:polishing a surface of a titanium base alloy to achieve a Rmax of 0.7micrometers or less; baking the titanium base alloy in an inert gasatmosphere, thereby removing moisture from the surface of the titaniumbase alloy; and subjecting the titanium base alloy to heat treatment ata temperature within a range of 20° C. to 300° C. in a mixed gasatmosphere of oxygen gas and 100 ppm of ozone gas, thereby forming anoxidation passive layer containing titanium oxides.
 21. The method forforming oxidation passive layers according to claim 20, wherein themixed gas further comprises hydrogen gas in an amount of 10 percent orless.
 22. The method for forming oxidation passive layers according toclaim 20, wherein the titanium base alloy contains Ti in an amount of 99percent by weight or more, Fe in an amount of 0.05 percent by weight orless, C in an amount of 0.03 percent by weight or less, Ni in an amountof 0.03 percent by weight or less, Cr in an amount of 0.03 percent byweight or less, H in an amount of 0.005 percent by weight or less, O inan amount of 0.05 percent by weight or less, and N in an amount of 0.03percent by weight or less.
 23. A stainless steel component comprising:stainless steel having an oxidation passive layer, said oxidationpassive layer having a thickness or 3 nm or more, said being polished toa Rmax of 0.7 micrometers or less prior to formation of said oxidationpassive layer thereon; and said oxidation passive layer having anoutermost surface composed of aluminum oxides.
 24. The stainless steelcomponent according to claim 23 wherein the stainless steel contains Alin an amount within a range of 0.5 percent by weight and 7 percent byweight.
 25. The stainless steel component according to claim 24 whereinthe stainless steel contains Al in an amount within a range of 3 percentby weight and 6 percent by weight.
 26. The stainless steel componentaccording to claim 23 wherein said oxidation passive layer comprises amixed oxide layer of aluminum oxides and chromium oxides.
 27. Thestainless steel composite according to claim 23, further comprising: afluid supply system in contact with ozone, said fluid supply systemcomposed of said stainless steel.
 28. The stainless steel compositeaccording to claim 23, further comprising: a fluid-contacting portion incontact with ozone, said fluid-contacting portion composed of saidstainless steel.
 29. A titanium base alloy component comprising:titanium base alloy having an oxidation passive layer, said oxidationpassive layer having a thickness of 3 nm or more; and said oxidationpassive layer having an outermost surface composed of titanium oxidescomponents.
 30. The titanium base alloy component according to claim 29wherein said outermost surface is polished to a Rmax of 0.7 micrometersor less prior to formation of said oxidation passive layer thereon. 31.The titanium base alloy component according to claim 29 wherein thetitanium base alloy contains Ti in an amount of 99 percent by weight ormore.
 32. The titanium base alloy component according to claim 29wherein the titanium base alloy contains Ti in an amount of 99 percentby weight or more, Fe in an amount of 0.05 percent by weight or less, Cin an amount of 0.03 percent by weight or less, Ni in an amount of 0.03percent by weight or less, Cr in an amount of 0.03 percent by weight orless, P in an amount of 0.005 percent by weight or less, O in an amountof 0.05 percent by weight or less, and N in an amount of 0.03 percent byweight or less.
 33. The titanium base alloy composite according to claim29, further comprising: a fluid-contacting portion in contact withozone, said fluid-contacting portion composed of said titanium basealloy.
 34. A method for forming oxidation passive layers, comprising:subjecting stainless steel containing Al in an amount within a range of0.5 percent by weight to 7 percent by weight to heat treatment within arange of 300° C. to 700° C. in a mixed gas atmosphere of an inert gasand H₂O gas in an amount within a range of 500 ppb to 1 percent,thereby, forming an oxidation passive layer containing aluminum oxideson a surface of said stainless steel.
 35. The method for formingoxidation passive layers according to claim 34, wherein the mixed gasfurther comprises hydrogen gas in an amount of 10 percent or less.
 36. Amethod for forming oxidation passive layers, comprising: polishing asurface of stainless steel containing Al in an amount within a range of0.5 percent by weight to 7 percent by weight to achieve a Rmax of 0.7micrometers or less; baking the stainless steel in an inert gasatmosphere, thereby removing moisture from the surface of the stainlesssteel; and subjecting the stainless steel to heat treatment at atemperature within a range of 300° C. to 700° C. in a mixed gasatmosphere of an inert gas and H₂O in an amount within a range of 500ppb to 1 percent, thereby forming an oxidation passive layer containingaluminum oxides on the surface of the stainless steel.